The present invention relates to the circuits for use with a phase cut dimmer, and in particular to an arrangement where the range of a phase cut dimmer is learned over time and utilized to improve user control of an LED luminaire.
Solid state lighting, and in particular light emitting diodes (LEDs) are rapidly coming into wide use for lighting applications. In most general lighting applications the LEDs are supplied in one or more strings of serially connected LEDs sharing a common current.
LEDs providing high luminance exhibit a range of forward voltage drops, denoted Vf, and their luminance is primarily a function of current. Brightness control of the LEDs may be performed by either pulse width modulation (PWM) or by amplitude modulation. In a PWM brightness control a fixed current is driven through the LED string, and the duty cycle of the fixed current is adjusted in order to control the LED string brightness. In amplitude modulation the amount of current through the LED string is varied directly, thus adjusting the brightness. LED strings exhibit a particular voltage to current relationship, wherein for a voltage below a minimum operating voltage no appreciable current flows, and for voltages exceeding the minimum operating voltage the current follows an exponential curve responsive to the voltage.
A phase cut dimmer is a device arranged to provide control of the brightness of a lighting source by blocking a portion of the alternating current (AC) mains power sine wave from reaching the lighting source. Both leading edge dimmers, wherein the leading edge of the sine wave is blocked by a settable conduction angle, and trailing edge dimmers wherein a trailing edge of the sine wave is blocked are commercially available. Other phase cut dimmers which allow selection of the portion of the sine wave to pass are also known. Phase cut dimmers are typically implemented by thyristors which require a minimum holding current, denoted Ih to operate smoothly, and exhibit a phase delay angle, denoted herein as phase cut angle φ.
Phase cut dimmers exhibit a range of phase cut angles φ, which may vary between models, and even between phase cut dimmers of the same model type, particularly in the event that the minimum holding current is supplied by the LED luminaire driver. In particular, a phase cut dimmer is typically unable to pass 100% of the AC mains power sine wave, and typically does not exceed a maximum of 90% of the AC mains power sine wave. Similarly, phase cut dimmers are typically unable to pass less than 10% of the AC mains power sine wave, since the phase cut dimmer is connected serially with the AC mains voltage and thus block a certain percentage of AC mains power sine wave.
LED lighting typically requires a constant current power source, and is thus preferably isolated from the direct action of the phase cut dimmer. What is desired is a means of utilizing a phase cut dimmer to control the brightness of an LED based luminaire in a manner wherein the LED based luminaire brightness is controlled over the entire range of achievable brightness responsive to the actually installed phase cut dimmer.